Circular Arrangement Research Articles

Task-irrelevant filler items alter the dynamics of electrical brain activity during visual search.

In electroencephalography (EEG) studies of visual search, task-irrelevant fillers are included in displays to balance bottom-up stimulation across the visual field and generally considered as inconsequential for performance or EEG results. We examined the impact of fillers on target and distractor processing using lateralised event-related potentials (ERPs). Two task-relevant items (TRIs) were presented, with or without fillers. One TRI (target or target-colour distractor) was on the vertical midline and the other in a lateral position (left or right visual field) on an imaginary circle around fixation. An N2pc was elicited by lateral targets and task-relevant distractors, suggesting that attention was allocated to the lateral TRI because it possessed a target defining feature (colour). A Ptc was only elicited by lateral task-relevant distractors, in line with previous research suggesting that this component is associated with distractor processing. When fillers were also in the circular arrangement, alterations in performance and neural activity occurred. Fillers enhanced and delayed attentional deployment (N2pc) and delayed distractor processing (Ptc). Critically, we observed no difference in Ptc amplitude according to filler presence. Thus, if the Ptc reflects active suppression (or attentional disengagement), it was not required for fillers. ERPs were also modulated by the distance between TRIs (which could be separated by one or four filler positions), but differently according to the colour scheme (blue TRIs with grey fillers or vice versa). Our results suggest that fillers affect lateralised electrophysiological activity at multiple loci during visual search and should not be considered inconsequential.

Searching on the Back: Attentional Selectivity in the Periphery of the Tactile Field

Recent evidence has identified the N140cc lateralized component of event-related potentials as a reliable index of the deployment of attention to task-relevant items in touch. However, existing ERP studies have presented the tactile search array to participants' limbs, most often to the hands. Here, we investigated distractor interference effects when the tactile search array was presented to a portion of the body that is less lateralized and peripheral compared to the hands. Participants were asked to localize a tactile target presented among distractors in a circular arrangement to their back. The N140cc was elicited contralateral to the target when the singleton distractor was absent. Its amplitude was reduced when the singleton distractor was present and contralateral to the target, suggesting that attention was directed at least in part to the distractor when the singletons are on opposite sides. However, similar N140cc were observed when the singleton distractor was ipsilateral to the target compared to distractor absent trials. We suggest that when target and singleton distractor are ipsilateral, the exact localization of the target requires the attentional processing of all items on the same side of the array, similar to distractor absent trials. Together, these observations replicate the distractor interference effects previously observed for the hands, suggesting that analogous mechanisms guide attentional selectivity across different body parts.

Performance Development and Evaluation of Solar Air Collector with Novel Phase Change Material

Experiments were carried out on an evacuated tube solar air collector connected to intrinsic thermal power storage to provide warm air unless solar radiation was available. As a phase change material, stearic acid was employed (PCM). Water has been used as a base fluid for converting sunlight into electricity gain to warm air, and the solar collector’s manifold was connected to the intrinsic thermal energy store. The most significant temperature variation between warm air and ecologic air was 38°C and 22°C, respectively, during direct and indirect solar radiation. A circular fin arrangement was used to achieve a flow rate of 0.020 kg s-1. The efficiency of minimum airflow rates (0.020 kg s-1) was 0.08–0.48 times that of maximum airflow rates (0.04 kg s-1). Because of the PCM’s better heat-storing capability, this system has a benefit over sensible storage systems in that it may be used after sunset.

Prominent Pseudoacini in Focal Nodular Hyperplasia: A Potential Diagnostic Pitfall.

Pseudoacini are generally a morphologic feature of hepatocellular carcinoma (HCC), being absent or rare in benign hepatocytic tumors, such as hepatocellular adenoma. However, rarely these can be seen in focal nodular hyperplasia (FNH) and may pose diagnostic challenges, especially when prominent. The study was aimed to evaluate the occurrence of pseudoacini in FNH and their clinicopathologic correlations. A total of 95 FNH cases diagnosed from 2005 to 2020 were included in the study. A pseudoacinus was defined as a circular arrangement of hepatocytes around a central dilated lumen present within the lobular parenchyma of the lesion with or without inspissated bile. Among the 95 FNH cases, 28 (29.5%) showed pseudoacini, which were prominent in 12 (12.6%) cases. Of these 3 occurred in patients above 50 years old. The pseudoacini were numerous in 3 cases, leading to an initial consideration of HCC in the differential diagnosis, and 1 case was diagnosed as well-differentiated hepatocellular neoplasm on initial biopsy. All 12 cases showed map-like staining pattern for glutamine synthetase. The hepatocytes forming the pseudoacini were positive for CK7 and HepPar1, while the inner lumina were highlighted by CD10 and bile salt export pump immunostains similar to adjacent canaliculi. The presence of prominent pseudoacini was not significantly associated with any clinical or pathologic features. The findings suggest that pseudoacini are likely manifestation of hepatocyte biliary transdifferentiation associated with chronic cholestasis in the lesion. This feature may pose a potential diagnostic pitfall especially on needle biopsies and awareness is needed to avoid misdiagnosing this as HCC.

Раннесарматские погребения могильника Верхний Балыклей I из Заволжья

The paper introduces into scientific discourse the materials of the early Sarmatian burials from the Verkhniy Balykley I kurgan cemetery from the area of the Volgograd Trans-Volga region. This monument was studied in the course of the 1978–1979 expedition organized by the Volgograd State Pedagogical Institute under the leadership of A.S. Skripkin. Materials. The burial includes four mounds located compactly on the banks of the reservoir and one single mound setting far away in the steppe. Eighteen inlet burials of the early Sarmatian time were found in three Bronze Age kurgans. Some burials contained no burial inventory; others did include a variety of grave goods. The funeral rites present a wide range of grave structures: pits with side chambers, simple rectangular pits and catacombs. The southern body orientation of the buried individuals was dominating. In kurgans 5 and 6, a circular arrangement of graves is traced, which is typical for early Sarmatian mounds-cemeteries. Material analysis. An analysis of the grave goods indicates 3rd – 1st centuries BC is the time of construction of most burials in these kurgans, which does not contradict the funeral rite traditions and reveals a range of features characteristic of the nomads of that period. A military burial with a long sword and arrows is one of the most distinguished in its kind. Ceramic material is represented by various stucco forms of pots, an incense burner and a jug. Fragments of mirrors with rollers along the edge and beads were discovered in several burials. Several stone “miniature hammers”, typical for the South Ural grave materials of this period, is also an interesting find. Conclusions. In general, the studied monuments illustrate the burial traditions of the early Sarmatian population of the Ural-Volga steppes of the early Sarmatian time. The finds of pottery samples, previously unknown stone “miniature hammers” in the west of the Volga, allow us to conclude on the cultural proximity of the nomads who left the mounds near the village Verhniy Balykley with the South Ural nomads. Authors’ contribution. In this paper, Anatoly S. Skripkin prepared a descriptive part of the archaeological material; Vladimir I. Moiseev analyzed the funeral rite and the inventory. Mikhail V. Krivosheev drew and presented conclusions on the analysis of the material.

Experimental Study on Fluid Elastic Instability of Concentric Circular Arrangement Heat Exchanger Tubes

In the industrial production, there exists the phenomenon of structure vibration caused by flow, which will cause the deformation and failure of the structure and lead to the economic loss, and even cause the industrial accident and great danger. In this paper, the one mechanism of fluid-induced vibration, namely fluid elastic instability, of the concentric circle arrangement tube bundles in the sodium-sodium heat exchanger is studied experimentally. According to different central angle positions, three kinds of tubular plates are designed, which are approximately triangle arrangement, approximately square arrangement and transitional arrangement respectively. In the small-size water tunnel non-contact graphic measurement is adopted to obtain the critical flow rate Ucr when the tube bundle is unstable. The experimental results show that the flow induced vibration resistance of the transitional arrangement is better, and the critical flow velocity is about 3.78.

Investigation of a new modulated aperture using speckle techniques

BackgroundA design of equally spaced eight-circles placed at equal distances from the origin is suggested. Three models corresponding to the eight-circle design considering conic, linear, and quadratic distributions are investigated. This arrangement is considered for the sake of improving both microscope resolution and image contrast as compared with the pure annular aperture. This design is different compared with other recent work on aperture modulation.Results and discussionsThe point spread function (PSF) is computed in all the models using the fast Fourier transform (FFT) algorithm that computes the discrete Fourier transform (DFT) corresponding to the models and compared with the corresponding PSF in the case of uniform circular aperture. In addition, the autocorrelation images for the apertures are shown differently. It is shown smooth pattern for the circular arrangement as compared with the deformation and shrinking appeared in the central peak in case of conic model. Finally, the speckle images corresponding to the considered apertures are investigated. Reconstructed apertures are obtained from the speckle images using the FFT algorithm.ConclusionsThe PSF is computed for the described models, and the autocorrelation corresponding to the apertures showed difference. The reconstructed apertures from the speckle images can be improved using filtering techniques. It is noted that MATLAB codes are constructed in the computations of all images and plots.

Robust independent and simultaneous position control of multiple magnetic microrobots by sliding mode controller

Recent development in technology and improvement of manufacturing tools have accelerated the use of microrobots (MRs) in numerous areas such as micro sensing and medical applications. The ability to control multiple MRs simultaneously and independently could lead to higher performance, and even make new applications possible. In this paper, we have proposed a system for simultaneous and independent control of the position of multiple MRs in a plane. The system consists of 2N permanent magnets (PMs) with a circular arrangement in the plane around the workspace and a pair of Helmholtz coil to control N MRs. PMs are rotated by servomotors, and the coil aligns the orientation of the MRs normal to the plane. A sliding mode controller, as a robust approach, is designed to eliminate the undesired effects of disturbances and uncertainties. For this purpose, at each step, the required force for each MR is calculated based on the proposed controller, and corresponding angles of the PMs are computed analytically. The simultaneous control of multiple MRs is illustrated by simulation, and the performance of the system is discussed. Also, the position control of three MRs with dimensions of 250 µm, and a maximum speed of 330 µm/s is experimentally demonstrated.

W-shaped slot-loaded U-shaped low SAR patch antenna for microwave-based malignant tissue detection system

This paper describes a wideband W-shaped slot-loaded solid U-shaped rectangular patch antenna for a microwave-based head imaging application. The proposed antenna comprises a W-shaped slot-loaded solid U-shaped radiating patch and a slotted partial ground. Slots have been used to enhance efficiency, current distribution, gain, and other antenna performances. At its first resonance frequency of 1.6 GHz, the optimized electrical dimension is 0.197λ × 0.298λ × 0.085λ. The antenna is printed on a low-cost 1.6 mm-thick FR4 substrate. The antenna feeds with a 50 Ω transmission line. The assessed findings demonstrated that the stated prototype has a 1.12 GHz (1.40 - 2.52 GHz) bandwidth along with omnidirectional radiation properties. The prototype achieved a maximum gain of 3.5 dBi at 2.44 GHz and provided a maximum radiation efficiency of 95% with stable radiation characteristics. The face-to-face alignment fidelity factor (FF) is also determined to evaluate time-domain responses above 80%. Near field directivity (NFD) is also analyzed with a head model to ensure near field radiation characteristics. The SAR value reaches a < 0.3 W/kg limit over the active bandwidth. Finally, in a CST simulation platform, an array configuration with nine antenna elements is deployed in a circular arrangement around the head phantom to collect the scattered data for tumor object characterization.

Ultrabroadband Circularly Polarized Antenna Array Based on Microstrip to SIW Junction

This communication presents a circularly polarized (CP) antenna array with ultrabroadband characteristics and a high gain for millimeter-wave applications. The CP is achieved by arranging eight linearly polarized (LP) tapered slot antenna elements in a circular arrangement based on sequential rotation (SR) techniques. The hybrid structure takes advantage of microstrip line (MSL) compactness in the feeding network and the capabilities of the substrate integrated waveguide (SIW) in antenna element design. A new T-junction MSL-to-SIW transition with a wide range of coupling ratios is used to serve as a wide power divider in the compact SR-feeding network. A detailed parametric study is carried out to demonstrate the effects of the coupling mechanism between the two transmission lines and to find the optimal configuration for the elements of the proposed antenna array. A tradeoff is made between antenna coupling, phase deviation, and overall size to achieve a compact structure with robust performances. An eight-element antenna array prototype is fabricated through the PCB process with a size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$57\times 48\times96$ </tex-math></inline-formula> mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> . Measured results show a 3 dB axial ratio bandwidth of about 49% over the entire 22.8–37.7 GHz range and a maximum gain of 18 dBi.

A Lotus shaped acoustofluidic mixer: High throughput homogenisation of liquids in 2 ms using hydrodynamically coupled resonators

This paper presents an acoustically actuated microfluidic mixer that uses an array of hydrodynamically coupled resonators to rapidly homogenise liquid solutions and synthesise nanoparticles. The system relies on 8 identical oscillating cantilevers that are equally spaced on the perimeter of a circular well, through which the liquid solutions are introduced. When an oscillatory electrical signal is applied to a piezoelectric transducer attached to the device, the cantilevers start resonating. Due to the close proximity between the cantilevers, their circular arrangement and the liquid medium in which they are immersed, the vibration of each cantilever affects the response of its neighbours. The streaming fields and shearing rates resulting from the oscillating structures were characterised. It was shown that the system can be used to effectively mix fluids at flow rates up to 1400 µl.min−1 in time scales as low as 2 ms. The rapid mixing time is especially advantageous for nanoparticle synthesis, which is demonstrated by synthesising Poly lactide-co-glycolic acid (PLGA) nanoparticles with 52.2 nm size and PDI of 0.44.

Dedicated SiPM array for GRD of GECAM

PurposeThe discovery of gravitational waves and gamma-ray bursts heralds the era of multi-messenger astronomy. With the adoption of two small satellites to achieve the all-sky monitoring of gamma-ray bursts, the gravitational wave high-energy electromagnetic counterpart all-sky monitor (GECAM) possesses a quasi-real-time early warning ability and plays an important role in positioning the sources of gravitational waves and in subsequent observations. Each satellite of GECAM was fitted with 25 3-inch-diameter gamma-ray detectors (GRD), covering an energy range of 8–2 MeV. GRDs have adopted silicon photomultiplier tubes (SiPM) in lieu of photomultiplier tubes (PMT) to adapt to the dimensional limitations of micro-satellites.MethodsA unique 3-inch circular SiPM array was designed. In this design, 64 6 × 6 mm chips were arranged evenly in a circular manner with the seams filled with reflecting films, thus achieving satisfactory uniformity of light collection. The integrated pre-amplifier circuit on the back of the SiPM array adopted two-level grouping and summing; further, it achieved a satisfactory signal-to-noise ratio. Two high-gain and low-gain channels were adopted to achieve a large dynamic range, and two independent power supply units were used, where each unit can be closed separately, thus improving reliability.ResultsPerformance studies show that this SiPM array meets the requirements of GECAM.ConclusionA 3-inch SiPM array have been developed that uses grouped summation, reflective films, a circular arrangement, two groups of independent power supplies, high- and low-gain signals, differential signal output technologies, etc. This solution can be used not only for GECAM, but also as a general solution for SiPM-based scintillation detectors.

A novel few-views arrangement of the fixed X-ray tubes for tomosynthesis.

Tomosynthesis is a technique that reconstructs a volume image from limited-angle projection data. In conventional tomosynthesis, the examination time is long, so it can be difficult for patients to hold their breath during certain examinations, such as chest imaging. Few-views tomosynthesis, which uses a linear arrangement of fixed X-ray tubes and enables an image to be obtained within 1 s, was found to be useful in the clinical setting in our previous study. In the present study, we attempted to develop a novel few-views tomosynthesis system that can obtain images with an improved image quality. A novel few-views arrangement of X-ray tubes was proposed and the image reconstruction method with regularization term was applied. The linear arrangement was used for the X-ray tube arrangement in our previous few-views tomosynthesis, in contrast, a circular arrangement was proposed in this study. The validation of this system was conducted with a numerical simulation and a real data experiment. The wider the scan angle, the more the object shadow spreads from "in-plane", allowing for artifact suppression. In the circular arrangement, the constant scan angle of θ is used, but in the linear arrangement the scan angle is set from 0 to θ. The artifacts in "out-of-plane" were more strongly suppressed in the circular arrangement than in the linear arrangement. Artifacts spreading in the z-direction were more strongly suppressed using the circular arrangement than the linear arrangement. Therefore, the circular arrangement was deemed appropriate for few-views tomosynthesis.

Integrated optical phased arrays with circular architecture on a silicon platform

Optical phased arrays (OPAs) are now at the forefront of photonic research as a key beam steering technology for myriad of photonic applications, including in light detection and ranging (LIDAR), communications, and metrology, among others. Integrated OPAs with narrow beam widths and wide-angle steering are in critical need, especially for LIDARs in autonomous vehicle, drone and airplane navigation, or satellites. In this work, we numerically study the performances of OPAs having a circular layout arrangement. Compared to recently available solutions with 1D linear or 2D rectangular arrays, the proposed circular OPAs are poised to deliver effective suppression of the grating sidelobes, while improving beam steering range and obtaining narrower beamwidths. We demonstrate 110-element circular arrays with sidelobe suppression better than 10 dB and an angular beamwidth of 0.5°. Under a monochromatic operation at a 1550 nm wavelength, such array provides a solid angle steering range of 0.21π-sr, with a perspective for performance improvement by using large number of OPA elements and operating under broader spectral range.

Аварийные раскопки на могильнике Маяк-1

As a result of work on located near the Altai region Krasnoshchekovskij district Mayak-1 burial ground in 1997 it was established that «mound 3» in group 1 consisted of IV–III centuries BC three mounds attached to each other in triangular shape. According to all data there was a deliberate arrangement of three cultural groups burials: 1) Pazyryk people (burial mound 3b); 2) Pazyryk people who perceived Kamenskaya culture burial rite with mound device of several burials around the central (mound 3b); 3) unknown cultural affiliation population with a circular arrangement of graves around the central one, with the burial of a man in undercut grave, and a full horse carcass in the entrance pit of the grave (mound 3a). The materials indicate that this foothills area was the contact one where Pazyryk population and adjacent plain inhabitants mixing and acculturation took place

CFD model for tubular SOFC stack fed directly by biomass

The energy transition can also be promoted by the sustainable use of biomass. Residual biomass in the Mediterranean areas can be exploited to a greater extent through highly efficient fuel cell systems. The Direct Biomass-SOFC project is based on a direct coupling between biomass power supply and SOFC tubular cells. This research project stems from the need to cover the electricity demand, avoiding the use of non-renewable sources. It will be investigated the unused or little-used biomass sources that can be exploited from the Mediterranean area.To this purpose, analyses were conducted to model a SOFC tubular cell stack by investigating the optimal configuration. The basic objective is to design a SOFC tubular cell stack, fed by syngas to produce at least 200 W. Two configurations were chosen: a square and a circular arrangement. Another objective of the study is to choose the best temperature control system. It have been selected a pressurised water system and an air system. The results show that the best performance is guaranteed by a square arrangement with an air temperature control system. The circular configuration provides less power than the square configuration, being limited by the multiple series connection to the lowest current value. The maximum electrical power produced with the square configuration is 225 W.

Video registration of physicochemical processes in BOF cavity at bath top blowing at application oxygen lances of various designs. Report 3. The picture of bath blowing at application two-level oxygen lances

Further increase of resources- and energy-saving efficiency of BOF processes is unthinkable without development of new methods of blowing and designs of blowing devices. It requires information on the real physicochemical phenomena in the converter cavity accompanying the blowing of the converter bath using new designs of oxygen lances in order to assess the possible risks in the mastering of the proposed developments in industrial conditions. The paper presents the results of video filming of the top blowing of a 80-kg converter bath by groups of multi-pulse supersonic and sonic oxygen jets formed, respectively, by Laval and cylindrical two-level nozzles of two designs equipped with double-row tips with a circular arrangement of Laval nozzles and cylindrical ones and upper block with cylindrical nozzles. Previously unknown information was obtained on the picture of the bath blowing with the formation of a reaction zone of interaction of supersonic and sonic oxygen jets with a metal melt with a flow of carbon monoxide going out the bath and afterburning of CO to CO2 under conditions of a counter-directed double curtain of sonic oxygen jets at different levels of location of the foamed slag-metal emulsions. It was established that in the initial period of blowing during slag formation most of the thermal energy of CO to CO2 combustion flares is transferred to the surface of the bath with lumps of added lime, and the rest is transferred by forced convection to the converter walls and gases escaping from the bath to the neck. In the case of the location of the foamed slag level at the upper tier of the cylindrical nozzles of the lance, heat transfer from high-temperature flares of localized afterburning of CO to CO2 within a limited in size near-lance flow of exhaust gases from the reaction zone is carried out according to the laws of submerged combustion and is completed completely in foamed slag-metal emulsion with the prevention of aggressive action of afterburning flares and volumes of overheated slag on the converter lining. Revealed and recorded by video recording modes of blowing the converter bath, contributing to the development of such undesirable phenomena during smelting as the appearance of intense emissions of slag-metal suspension from the facility, coagulation of the slag with the cessation of dephosphorization of the metal melt, the development of intense dust formation and the removal of small metal particles and slag with the formation of crust on the lance barrel. A variant of the final stage of blowing with a transition to supplying nitrogen instead of oxygen through cylindrical nozzles of two-level lances was experimentally tested, which provides an effective reduction in the level of foamed slag-metal emulsion before the converter turning down. The data obtained were used in the development of an industrial design of a two-level lance with a double-row tip, blowing and slag modes of blowing a converter bath with its use.

Numerical study of the fluid dynamics and heat transfer for shear-thinning nanofluids in a micro pin-fin heat sink

The ability to enhance heat transfer rates of shear-thinning fluids in microchannel devices is evaluated numerically for steady and time-dependent flows in the range of 400 < Re < 2000. The geometry used represents a simplified micro pin-fin heat sink device with a staggered circular pin arrangement. The working fluids are composed by water and ethylene-glycol with different kinds of nanoparticles and concentrations. Four Newtonian and five non-Newtonian nanofluids are evaluated in detail, focusing on their rheological behavior, the heat extraction capacity, and the fluid dynamics developed for each flow condition. The nanofluids studied are extracted from experimental articles, and they are characterized as shear-thinning power-law ones with power-law indexes in the range 0.4946 <n < 0.69. Comparisons of heat-flux between the inlet and the outlet of the microchannel are investigated for nine different Reynolds numbers. Additionally, the pressure drop was evaluated for each case. The results show that the shear-thinning behavior of the nanofluids is the most critical factor in enhancing heat transfer rates due to the promotion of unsteady flows even for low Reynolds number values and a reduction of pressure drop. A large number of numerical tests are presented and carefully analyzed to justify our claims.

Comparison of Different Fin and Tube Compact Heat Exchanger with Longitudinal Vortex Generator in CFU-CFD Configurations

Over the last decades several studies have searched for improved Fin and Tube Heat Exchanger (FTHE) designs capable of providing the best thermo-hydraulic performance. The present study aims at quantifying and comparing the thermo-hydraulic performance of different FTHE configurations. Six different designs were analyzed. The first FTHE consisted of an in-line circular tube arrangement and the last one was a FTHE with staggered oval tube with two pairs of Delta Winglet Vortex Generators (DWVG) in common flow up–common flow down (CFU-CFD) configuration. The best performance was obtained using DWVG in CFU-CFD orientation. This configuration enabled a 90% increase of the thermal performance factor when compared with the first case, using only two pairs of vortex generator´s per tube.

Aerodynamic Shape Optimization Method of Non-Smooth Surfaces for Aerodynamic Drag Reduction on A Minivan

To reduce aerodynamic drag of a minivan, non-smooth surfaces are applied to the minivan’s roof panel design. A steady computational fluid dynamics (CFD) method is used to investigate the aerodynamic drag characteristics. The accuracy of the numerical method is validated by wind tunnel test. The drag reduction effects of rectangle, rhombus and arithmetic progression arrangement for circular concaves are investigated numerically, and then the aerodynamic drag coefficient of the rectangle arrangement with a better drag reduction effect is chosen as the optimization objective. Three parameters, that is, the diameter D of the circular concave, the width W and the longitudinal distance L among the circular concaves, are selected as design variables. A 20-level design of an experimental study using a Latin Hypercube scheme is conducted. The responses of 20 groups of sample points are obtained by CFD simulation, based on which a Kriging model is chosen to create the surrogate-model. The multi-island genetic algorithm is employed to find the optimum solution. The result shows that maximum drag reduction effects up to 7.71% can be achieved with a rectangle circular concaves arrangement. The reduction mechanism of the roof with the circular concaves was discussed. The circular concaves decrease friction resistance of the roof and change the flow characteristics of the recirculation area in the wake of the minivan. The roof with the circular concaves reduces the differential pressure drag of the front and rear of the minivan.